1. Field of the Invention
This invention relates to zoom lenses of the rear focus type.
2. Description of the Prior Art
The method called "rear focusing" or "inner focusing" has been found useful for quick and easy focusing in many intermediate to super telephoto lenses. This focusing method avoids the problem of telephoto lenses having relatively long physical lengths and heavy front lens components. There, focusing by moving the entire system as a whole or by moving the front component results in difficult rotation of a helicoid or in a change of the balance of the entire lens system as the focusing component moves forward.
The rear focusing method was invented in order to overcome such drawbacks and performs its focusing through lens elements which are relatively near the film plane and are of small diameter.
In this rear focusing method, an appropriate power distribution in the focusing component makes it possible to avoid deterioration of the optical or imaging performance and also to reduce the amount of movement of the focusing component to the closest object.
This has many advantages, particularly during automatic focusing. The focusing component is easy to drive by virtue of its light weight and compact form. The signal transmission from the camera body assumes a simple form because the focusing component lies relatively near the camera body. The feedback for detection of the in-focus condition is easy to apply because the total focusing movement can be minimized.
These advantages are also found in zoom lenses. With a prior zoom lens, particularly one which is focused by driving the front component, raising the zoom ratio or increasing the relative aperture requires increasing the entire lens system in both the longitudinal and lateral directions. The rear focusing method is very effective for satisfying the requirement of size and convenience. However, the employment of the rear focusing method in a zoom lens generally requires providing the lens system with an air separation through which the focusing component operates over the required range. This tends to elongate the total length of the zoom lens.
Japanese Laid-Open Patent Application No. SHO 57-78514 deals with a compact and well-corrected zoom lens. It discloses a lens system that includes, from front to rear, a stationary front group, magnification power-variable lens groups movable for zooming, and a rear group which as a whole does not change its position and which has a diaphragm in the interior thereof. The rear group has a negative-power lens member which is movable for focusing and located on the image side of the diaphragm. The movable lens member is moved rearward to focus down on shorter object distances. However, even this zoom lens cannot be said to be sufficient from the standpoint of compactness and aberration correction.